1. Field of the Invention
The present invention generally relates to composites and more particularly to composites of thin copper foil and plastic, which composites are useful in the manufacture of electrical and electronic components and the like.
2. Prior Art
Copper foil that has been laminated to plastics is commonly used in the manufacture of printed circuit boards. These boards are employed in a wide variety of commercial electronic applications such as in television sets, radios, computers, and automobile dashboards. In such applications it has been unnecessary to design printed circuits with conducting elements smaller than about 1/16 inch in width or with gaps between adjacent conducting elements smaller than about 1/32 inch. Such coarse dimensions have not required great precision in the preparation of the printed circuits nor much uniformity in the width of their conductors. Special extra-thin copper foil weighing 1/8 oz. to 1/2 oz. per sq. ft. is now made using processes that employ temporary carriers such as metal foils and plastics.
Typically, however, conventional copper foil clad laminates are made using copper foil that weighs between about 1/2 oz. per sq. ft. and about 2 oz. per sq. ft. and that has been formed by electrodeposition of copper onto the surface of a rotating drum. The drum surface has been pretreated so that the thus formed copper foil layer can be easily peeled away. The side of the thus formed copper foil, which is exposed, that is, is away from the drum, is much rougher than the side facing the drum and it is this rough side which is then bonded to plastic to form the typical composite or laminate referred to above.
Before the lamination step, however, the rough (matte) surface of the copper foil is usually treated to form thereon a plurality of small projections or nodules which enhance the bondability of the foil to the plastic. Thus, these nodules or projections comprise a myriad of microscopic particles of copper and/or copper oxide, some of which are only loosely adherent to the copper foil surface.
One such plastic adhesion-increasing treatment which causes formation of the nodules is described in U.S. Pat. No. 3,220,897, issued Nov. 30, 1965 to C. C. Conley. An improvement of that method is disclosed in B. Luce's U.S. Pat. No. 3,293,109, issued Dec. 20, 1966. In the latter patent, an encapsulating layer of copper is placed over and around the nodules or projections of copper-copper oxide particles (formed as by the method of U.S. Pat. No. 3,220,897) so as to lock those particles more securely to the matte surface of the copper foil.
Although both of the described patented methods are still used to manufacture copper foil for printed circuit laminates, problems are encountered as a result of their use. Thus, when the treated copper foil surface is brought into contact with the uncured plastic under high heat and high pressure during the lamination step, the copper or copper oxide particles tend to be broken away from the copper foil surface and to become embedded below the plastic surface. When the copper is then etched away, as during the preparation of the printed circuit, there remains behind a discoloration of the plastic surface, which is called "staining". Such discoloration comprises a plurality of the copper and/or copper oxide particles as embedded in the plastic, and adversely affects the dielectric properties of the plastic. Thus, the over-all performance of the printed circuit is impaired as well as its physical appearance.
More recent applications of copper foil clad laminates for printed circuits have been in areas requiring miniaturization of the printed circuits. New electronic components such as light emitting diodes and miniature assemblies of semiconductors, such as integrated circuits, have created more severe performance requirements for the manufacturers of printed circuits. Printed circuits with conducting elements having a width of only a few thousandths of an inch and spacings between elements of similar dimensions, have become common and have shown up the deficiencies of the ordinary copper foil adhesion-increasing treatments.
In order to insure that a copper conducting element of only a few mils width remains firmly attached to the plastic substrate during the fabrication of the printed circuit, the described adhesion-increasing treatment must be extensive and it must also be uniform. Despite such precautions, more numerous occurrences of "staining" or "brown spotting" usually are experienced as copper conductor widths are reduced. Moreover, an additional difficulty arises in the form of a phenomenon called "line lifting", where the conducting elements tend to separate over small areas from the plastic substrate. Examination of the separated areas under the lifted conducting elements often shows that the particular area of copper surface so affected has not been sufficiently nodularized.
An improvement in the treatment of copper foil surfaces to enhance bonding and reduce "staining" has been described by Luce in U.S. Pat. No. 3,585,010 issued June 15, 1971. In that patent, the encapsulating copper layer called for in U.S. Pat. No. 3,293,109 is replaced by a layer of one of a selected few other metals, namely, indium, nickel, tin, cobalt, bronze, zinc or brass. The layer is electroplated over the copper-copper oxide particles on the treated foil surface to form an encapsulating barrier layer. Some reduction in "staining" is often noted when this method is employed.
However, because of the substantial differences in the chemical characteristics of the metals indium, zinc and brass, from those of the copper over which they are deposited, certain other substantial problems are introduced when this method is used.
The zinc, whether present in the form of the pure metal or as the alloy brass, is highly soluble in conventional ferric chloride and copper chloride etching solutions used in the manufacture of the circuit boards, and, thus, is attacked much more rapidly during etching than is the copper. Therefore, when the excess copper being etched away to form the conducting elements, the zinc-rich layer that encapsulates the copper-copper oxide particles is exposed to the chloride etching solution and is stripped away quite rapidly. Since this encapsulating layer is at the bonding interface between the copper foil and the plastic, its more rapid attack results in undercutting of the conducting element.
If the conducting element is narrow, such as in the more recent circuit designs, then rapid etching of the small zinc-rich bonding interface severely reduces the bond between the copper conducting element and the plastic. This often results in low peel strength and/or line lifting. This susceptibility of the zinc to the chloride type etchants is well known and it is for this reason that zinc or brass (encapsulated) treated copper foil is not often used for fine line circuits that are to be etched in chlorides. Moreover, copper foil with such zinc-rich layer on its bonding surface requires special handling to prevent its dezincification, which can result even from casual contact with fingerprints.
The other metals listed in the Luce patent, have even greater drawbacks as stain resistant barrier layers. Indium, cobalt, and nickel, for example, have etching characteristics that are markedly different than copper and if used commercially they would require specialized etching procedures of two or more steps and different solutions to remove the composite metals.
Accordingly, there is a need for an improved copper foil for use in printed circuit laminate manufacture, which foil readily adheres to but reduces both staining of the plastic substrate and fine line lifting of copper conductor (made from the foil) from the substrate during such manufacture. There is also a need for an improved laminate of copper foil and plastic substrate having such properties and which can be processed into printed circuits without having to resort to special etching solutions.